Porosity refers to how susceptible your hair is to water: essentially, to what degree the outer layer of the strand takes in or keeps out water.

 “The outer layer of the hair strand is called the cuticle layer. The cuticle layer is made of little tiny cuticles that lie slightly over one another,” hairstylist Danielle Malary explains. Think of this as you would a shingles on a roof—and your porosity comes from how tightly those shingles are packed together.

“Hair porosity describes how the hair’s cuticle absorbs and holds on to moisture in its pores—hence, the term porosity,” says hairstylist Miko Branch, co-founder of hair care brand Miss Jessie’s. So when your hair high on the porosity spectrum, it means it’s very porous, and thus the cuticles are more sparse than they are dense, so it’s able to absorb a lot of water. 

In contrast, you can have low porosity hair: “When someone has low porosity hair, the hair tends to have trouble absorbing moisture because the hair is resistant to water,” says Malary. “When the hair has low porosity, the cuticles are packed so tightly onto each other, it makes it very difficult for moisture or water to penetrate the strand.”